1. Field of the Invention
The invention relates to a material for the metal components of high-temperature fuel cell systems, which are provided with ceramic solid electrolytes being formed of yttrium-stabilized zirconium oxide.
High-temperature fuel cell systems, which are also known as solid oxide fuel cell (SOFC) systems, are suitable for converting not only hydrogen gas but also hydrocarbons, such as natural gas or liquid-storable propane, because of relatively high operating temperatures which are in the range from 800.degree. to 1100.degree. C., in contrast to low-temperature fuel cell systems. If carbon dioxide and water vapor are added to the fuel, then at the high temperatures resulting from fuel conversion, it is possible to avoid any soot formation. In such high-temperature fuel cell systems, solid electrolytes are used for temperature reasons. In the case of such applications, it is known to insert ceramic solid electrolyte plates between the electrodes, with the plates substantially being formed of zirconium oxide and small amounts of additives, such as yttrium oxide. In known constructions, the solid electrolyte plates, along with electrodes on both sides and with the interposition of electrically conductive window foils, are connected to so-called bipolar plates which have good electrical conductivity, and have a grooved surface structure which assures the inflow of fuel and oxidizer. A number of such fuel cells is then stacked one above the other and therefore electrically connected in series, thus forming a fuel cell module or stack. A plurality of such stacks can then be assembled to make fuel cell systems.
Due to the high operating temperature, the ceramic solid electrolyte plates are exposed to severe mechanical strains when the fuel cell system heats up upon startup of operation or cools down again to room temperature after operation is turned off and the other components, such as bipolar plates and window foils contacting the solid electrolyte plates, also have coefficients of thermal expansion that differ only slightly from one another. Such strains can cause cracking in the solid electrolyte plate and shorten the service life of the high-temperature fuel cell module considerably.
German Published, Non-Prosecuted Application DE 40 09 138 Al discloses a solid electrolyte high-temperature fuel cell module in which window foils and bipolar plates contacting solid electrolyte plates made of yttrium-stabilized zirconium oxide, are made of a chromium-nickel alloy with nickel contents of from 5 to 15 weight %, or an iron-chromium-aluminum alloy with contents of 5 to 15 weight % of molybdenum and/or 5 to 15 weight % of tungsten. It is a characteristic of the first of those alloys that it is quite well adapted in its coefficient of expansion to the coefficient of expansion of the solid electrolyte. Unfortunately, its resistance to corrosion is not completely satisfactory. The situation for the second of those alloys is the reverse.